This invention relates to an improvement in the manufacture of powder paints, and more particularly in the manufacture of such paints wherein the raw batch ingredients comprising resinous binders and pigmentary solids are plasticized, homogenized, and formed into a coherent extrudate by mechanical extrusion ("hot extrusion process").
Powder paints are ostensibly dry ("hardened") and freeflowing at normal room temperature. They are applied to a substrate by conventional means such as electrostatic spray processes or fluidized bed processes. Because there are little or no fugitive components such as solvents or water in them, they must depend upon their own melting to coalesce, level out, and form a film. The absence in them of the fugitive components, however, is quite attractive in many industrial painting operations because atmospheric contamination from the volatile solvents, etc., is virtually, if not completely, eliminated. Powder paints are applied to hot substrates or those subsequently heated to generate the paint film.
Any single paint plant, including a powder paint plant, generally is expected to produce a wide variety of types and shades of paint in various volumes. Thus, the operators are working on a series of batch units, even though a particular batch unit or like batch units may be run in a continuous or semi-continuous manner for a restricted period of time. One of the biggest problems in making a powder paint is to obtain quite precise uniformity of color (shade) and other properties from batch unit to batch unit.
Accordingly one scheme has been to make the paint up like a traditional solvent-based paint, secure the shading, etc., then remove the solvent as by spray drying or by the process described in U.S. Pat. No. 3,737,401 (wherein the solvent is water soluble, and it is extracted by water from the rest of the paint which is then collected and dried). Thin film evaporation has also been proposed for such removal of solvent from a solvent paint preparatory to its comminution into a powder (generally passing 200 mesh U.S.S. sieve with a minimum of ultrafine material to suppress dusting).
Another process for producing powder paints is by hot extrusion of the raw batch components, which generally contain little or no volatile ingredients whatsoever. Such process is shown in U.S. Pat. No. 3,643,874. For the most part, color matching and adjustment of other critical compositional variables must be accomplished prior to such extrusion. This is especially true where thermosetting resinous vehicles are used in making such powder paints. Subjecting them to reprocessing with additional heat and/or prolonged time tends to advance their crosslinking, and this can result in prematurely hardened particles which are of little or no practical use as coatings. The thermoplastic vehicle powder paints are, of course, less susceptible to damaging from such reprocessing, but, nevertheless, it is expensive and they can become somewhat deteriorated.
A very real problem in the preparation of powder paints by the hot extrusion process is the problem of contamination between unlike batch units. Thus, even traces of acrylic resin-based particles from a previous batch appearing in an epoxy resin-based powder paint often can lead to visible specks or other imperfections which render the epoxy resin-based batch unsatisfactory. Hence, the problem of cleaning and maintaining all equipment in such extrusion process plants to a quite completely clean condition between unlike batch units is extremely critical, not only for such incompatible imperfections, but also for color matching of like batch units and color differentiation between unlike batch units.